Rotary helical splined actuators have been employed in the past to achieve the advantage of high-torque output from a simple linear piston-and-cylinder drive arrangement. The actuator typically uses a cylindrical body with an elongated rotary output shaft extending coaxially within the body, with an end portion of the shaft providing the drive output. An elongated piston sleeve is disposed between the body and the shaft and coaxially receives the shaft therein. The piston sleeve has a sleeve portion splined to cooperate with corresponding splines on the body interior and the output shaft exterior. The piston sleeve is reciprocally mounted within the body and has a head for the application of fluid pressure to one or the other opposing sides thereof to produce axial movement of the piston sleeve.
As the piston sleeve linearly reciprocates in an axial direction within the body, the outer splines of the sleeve portion engage the splines of the body to cause rotation of the sleeve portion. The resulting linear and rotational movement of the sleeve portion is transmitted through the inner splines of the sleeve portion to the splines of the shaft to cause the shaft to rotate. Bearings are typically supplied to rotatably support one or both ends of the shaft relative to the body.
While such an arrangement produces a relatively high torque output, the capability of the actuator is inherently limited by the use of splines, the splines having certain characteristic limitations relating to the number of turns, pitch, surface contact drag and free play between the corresponding splines which limit the axial and radial loads that the shaft can be subjected to and the operating efficiency of the actuator. For high-torque, high-efficiency applications, the rigid-splined helical actuators have been found to have an undesirably high frictional coefficient and large loads can cause binding between the corresponding spline. While actuators using balls to transmit torque have been manufactured and are an improvement upon the splined actuator, further improvement is desirable.
It will therefore be appreciated that there has been a significant need for a fluid-powered actuator and other devices, such as pumps, which are capable of handling increased axial and radial shaft loads while decreasing the weight and size of the device and decreasing the difficulty and expense of manufacturing the device. The device should have a low-friction design to increase efficiency, an uncomplicated and strong design to increase reliability, and be usable for heavy-duty applications relative to the size of the device. Preferably, the device will provide a higher output efficiency by using force transmission components producing rolling friction which is less than the sliding friction of splines. The device should also be manufacturable in a sufficiently smaller size so that it can be implanted within the human body and provide a wide rotational range. For such medical uses, an outer diameter size of 1.0 inch or smaller is desired. The device should also be usable for industrial applications and manufacturable in a wide range of sizes. The present invention fulfills these needs and further provides other related advantages.